This invention relates to spark plugs having dual gaps and polarization effects of dielectric, and more particularly to spark plugs in which such polarization effects take place between an end of a central electrode and earth electrode to improve discharge performance.
In recent years, such aspects as regulation of exhaust gas and improvement of fuel consumption rate have induced combustion in internal combustion engines to be effected under conditions which utilize lean fuel mixtures and high compression rate. Lean fuel mixtures and high compression rate are causes for worsening ignition of the fuel-air mixture. In this regard, spark plugs have been increasingly required to be improved in their ignition performance. Usually, it is well-known that ignition performance can be greatly improved by enlargement of discharge gaps. However, pressure rise in combustion chambers, in particular, pressure rise accompanying such high compression rate causes rise of discharge breakdown voltage, so that it is inconsistent with conventional ignition systems in terms of technical levels and cost, thus giving rise to a problem in not enabling enlarging gaps beyond some limits. Even if discharge gaps could be enlarged, there would remain a problem in which consumption of electrodes accompanies ignition systems of high energy.